Not Applicable
Not Applicable
1. Field of Invention
This invention relates to a detector which has the capability of operating in either positron emission tomography (PET) or x-ray computerized tomography (CT) mode. More specifically, it relates to a combined PET and CT scanner detector which allows a PET scanner and a CT scanner to utilize common detectors, resulting in better registration of the metabolic PET image with the anatomical CT image, fewer components in the gantry, and a reduction of the overall size and mass of the gantry.
2. Description of the Related Art
Various techniques are used for medical imaging. PET and CT are popular in radiology because of their ability to non-invasively study physiological processes and structures within the body. To better utilize PET and CT, recent efforts have been made to combine the two scanners into a single machine. This allows for better registration of the metabolic PET image with the anatomic CT image. The combined scanners share space on the same gantry, but use separate detectors,and associated hardware.
Positron Emission Tomography (PET) is a nuclear imaging technique used in the medical field to assist in the diagnosis of diseases. PET allows the physician to examine the whole patient at once by producing pictures of many functions of the human body unobtainable by other imaging techniques. In this regard, PET displays images of how the body works (physiology or function) instead of simply how it looks. PET is considered the most sensitive, and exhibits the greatest quantification accuracy, of any nuclear medicine imaging instrument available at the present time. Applications requiring this sensitivity and accuracy include those in the fields of oncology, cardiology, and neurology.
In PET, short-lived positron-emitting isotopes, referred to as radiopharmaceuticals, are injected into a patient. When these radioactive drugs are administered to a patient, they distribute within the body according to the physiologic pathways associated with their stable counterparts. For example, the radiopharmaceutical 18F-labeled glucose, known as fluorodeoxyglucose or xe2x80x9cFDG xe2x80x9d, can be used to determine where normal glucose would be used in the brain. Other radioactive compounds, such as 11C-labeled glucose, 13N-labeled ammonia and 15O-labeled water, are used to study such physiological phenomena as blood flow, tissue viability, and in vivo brain neuron activity.
As the FDG or other radiopharmaceutical isotopes decay in the body, they discharge positively charged particles called positrons. Upon discharge, the positrons encounter electrons, and both are annihilated. As a result of each annihilation event, gamma rays are generated in the form of a pair of diametrically opposed photons approximately 180 degrees (angular) apart. By detecting these annihilation xe2x80x9cevent pairsxe2x80x9d for a period of time, the isotope distribution in a cross section of the body can be reconstructed. These events are mapped within the patient""s body, thus allowing for the quantitative measurement of metabolic, biochemical, and functional activity in living tissue. More specifically, PET images (often in conjunction with an assumed physiologic model) are used to evaluate a variety of physiologic parameters such as glucose metabolic rate, cerebral blood flow, tissue viability, oxygen metabolism, and in vivo brain neuron activity.
Mechanically, a PET scanner consists of a bed or gurney and a gantry, which is typically mounted inside an enclosure with a tunnel through the center, through which the bed traverses. The patient, who has been treated with a radiopharmaceutical, lies on the bed, which is then inserted into the tunnel formed by the gantry. The gantry is rotated (either physically or electronically simulated with a stationary ring) around the patient as the patient passes through the tunnel. The rotating gantry contains the detectors and a portion of the processing equipment. Signals from the rotating gantry are fed into a computer system where the data is then processed to produce images.
The PET scanner detectors are located around the circumference of the tunnel. The detectors use a scintillator to detect the gamma rays. Suitable material used for the scintillator includes, but is not limited to, either lutetium oxyorthosilicate (LSO) or bismuth germanate (BGO). The light output from the scintillator is in the form of light pulses corresponding to the interactions of gamma rays with the crystal. A photodetector, typically a photomultiplier tube (PMT) or an avalanche photodiode, detects the light pulses. The light pulses are counted and the data is sent to a processing system.
Another known tomography system is computed axial tomography (CAT, or now also referred to as CT, XCT, or x-ray CT). In CT, an external x-ray source is caused to be passed around a patient. Detectors around the patient then respond to the x-ray transmission through the patient to produce an image of the area of study. Unlike PET, which is an emission tomography technique because it relies on detecting radiation emitted from inside the patient, CT is a transmission tomography technique which utilizes a radiation source external to the patient. CT provides images of the internal structures of the body, such as the bones, whereas PET provides images of the functional aspects of the body, usually corresponding to an internal organ or tissue.
The CT scanner uses a similar mechanical setup as the PET scanner. However, unlike the pairs of PET scanner detectors required to detect the gamma rays from an annihilation event, the CT scanner requires detectors mounted opposite an x-ray source. In third-generation computed tomography systems, the CT detectors and x-ray source are mounted on diametrically opposite sides of a gantry which is rotated around the patient as the patient traverses the tunnel.
The x-ray source emits a fan-shaped beam of x-rays which pass through the patient and are received by an array of detectors. As the x-rays pass through the patient, they are attenuated as a function of the densities of objects in their path. The output signal generated by each detector is representative of the electron densities of all objects between the x-ray source and the detector.
The CT detectors can utilize scintillator crystals which are sensitive to the energy level of the x-rays. Multiple light pulses produced by each scintillator crystal as it interacts with the x-rays are integrated to produce an output signal which is related to the number of the x-rays sensed by the scintillator crystal. The individual output signals are then collectively processed to generate a CT image. Other detectors can be used in CT tomographs. For example, a solid state silicon diode can be used to detect the low energy x-rays directly.
The medical images provided by the PET scanner and CT scanner are complementary, and it is advantageous to have images from both types of scans. To be most useful, the PET and CT images need to be overlaid or co-registered such that the functional features in the PET images can be correlated with the structural features, such as bones, tumors, and lung tissue, in the CT images. The potential to combine functional and anatomical images is a powerful one, and there has been significant progress in the development of multi-modality image co-registration and alignment techniques. However, with the exception of the brain, the re-alignment of images from different modalities is not straightforward or very accurate, even when surface markers or reference points are used. To this end, it is desirable to incorporate PET and CT scanners into a single gantry, thereby allowing the images to be taken sequentially within a short period of time and overcoming alignment problems due to internal organ movement; variations in scanner bed profile, and positioning of the patient for the scan.
In recent years, there has been considerable progress in the development of techniques to co-register and align functional and anatomical images. For example, David W. Townsend, et al., in their paper xe2x80x9cThe SMART Scanner: A Combined PET/CT Tomograph for Clinical Oncology,xe2x80x9d presented at the 1998 IEEE Nuclear Science Symposium and Medical Imaging Conference in Toronto, Canada, on Nov. 12-14, 1998, described a combined PET and CT tomograph. In this paper, the authors showed that a spiral CT scanner could be combined with a PET tomograph with the components mounted on a common rotational support within a single gantry. The PET and CT components could be operated either separately or in combined mode. The combined PET and CT scanner described in this article required a patient tunnel of 110 centimeters in order to accommodate the independent PET and CT components.
In P. E. Kinahan, et al., xe2x80x9cAttenuation Correction for a Combined 3D PET/CT Scanner,xe2x80x9d Med. Phys. 25 (10), 2046-53 (October 1998), the authors described their conception of a combined PET and CT scanner, in which the independent scanners were mounted on a rotating gantry. They identified the advantages of such an arrangement as including the application of CT-based attenuation correction to the PET scan and the use of CT data directly in the image reconstruction process.
Independent PET and CT detectors use numerous components which occupy space in the rotating gantry. This duplication of detectors results in inaccuracies in registration of the PET image with the CT image, a deeper patient tunnel, increased number of components in the gantry, increased mass on the gantry, and potentially higher cost.
Therefore, it is an object of the present invention to provide a common detector for a PET scanner and a CT scanner.
It is a further object of the present invention to improve the overlay registration of individual PET and CT images.
It is a further object of the present invention to reduce the number of components on the gantry and to reduce the depth of the tunnel.
Another object of the present invention is to detect a multitude of discrete, x-ray generated, scintillator events and provide count rate information for processing into a CT image by integrating the charge information from the multitude of x-ray events.
Still another object of the present invention is to count the individual x-ray events for a specified time to provide digital count rate information gathered from the common detector to generate a CT image.
In accordance with the teachings of the present invention, a common detector provides data which is processed to generate a PET image and a CT image. The common detector is mounted on a single support within the same gantry. The common detector includes an array of scintillator crystals or pixels, each pixel having a photodetector feeding both discrete event circuitry and integrating circuitry, both of which feed a processing system. A PET image and a CT image are generated sequentially by scanning a patient twice, once after the patent is injected with a radiopharmaceutical and once with an x-ray source rotated about the patient.
The common detector operates in three modes. The first mode of operation is as a standard PET detector, which detects individual gamma rays. When operating in this mode, the photodetector feeds a charge amplifier, which outputs a signal proportional to the energy level of a single detected gamma ray. In this mode, a fast timing signal is also derived. The second mode of operation is as a standard CT detector, which detects multiple x-rays over a short sample period. When operating in this mode, the photodetector feeds an integrator, which outputs an integrated signal proportional to the number of x-rays detected for the sample period. In the third mode of operation, the detector senses individual x-rays and outputs discrete event information which is processed into a CT image. In this mode, the photodetector feeds a fast amplifier, an energy discriminator, and a counter/timer circuit, which outputs a digital signal proportional to the number of x-rays detected in a preset time interval.
By using a common detector, the registration of the PET image with the CT image is improved, fewer components are mounted on the gantry, the overall size and mass of the gantry is reduced, and the tunnel the patient has to enter is shorter, reducing patient anxiety and claustrophobic effect. Also, the third mode of operation provides a digital signal to the CT scanner, which results in improved stability when compared to the analog method of the second mode of operation. tHE third mode also enables an output based on a specified range of x-ray energy, which provides an improvement in CT image contrast.